1. Field of the Invention
The invention relates to a multistage furnace in which a plurality of furnace units are piled up in the vertical direction.
2. Description of the Related Art
For vehicle components, a thinned and high-strength member is used so as to enhance both the safety and economy. For this purpose, so-called hot press is known in which a steel plate heated to high temperature is quenched by cooling the plate with low-temperature press dies. In this method, a steel plate is heated to transformation temperature or higher at which the metal structure of the steel member is transformed into austenite, and the steel plate is formed and rapidly cooled with press dies simultaneously, completing quenching.
As a furnace for hot press, as shown in FIG. 11, one is known in which a multiple number of lower heaters 1 and upper heaters 2 are arrayed in the horizontal direction and a steel plate W is carried and heated between the lower heaters 1 and the upper heaters 2 from the inlet to the outlet. Steel plates W are sequentially inserted into the furnace from the inlet thereof, and heated between the lower heaters 1 and the upper heaters 2 until these are discharged from the outlet. A relevant technique is disclosed in Japanese Patent Application Publication No. 2010-44875.
The conventional furnace heats a plurality of steel plates W sequentially, but the multiple number of lower heaters 1 and upper heaters 2 arrayed in the horizontal direction make the installation area of the furnace large in a factory. Furthermore, even when only one of the multiple number of lower heaters 1 and upper heaters 2 is broken due to burnout or the like, all the steel plates W inserted in the furnace become defective products due to underheating.